This is whar DxO says about the low light/high ISO capability of the G5 vs. the G3:

"However, the G5 delivered a lackluster low light performance. Its score lagged behind the already poor low light performance of the G3, contradicting Panasonic’s claims that the G5’s digital’ Live MOS sensor would improve upon its older brethren. "

While the differences may not be of much practical significance, the change from the G3 to the G5 (G3: ISO 667; G5: ISO 618) is in the wrong direction! By contrast, the OM-D E-M5's low light ISO score is 826. That's better than the Canon 650D (722).

Granted, there are many factors other than sensor performance--or any single aspect of sensor performance--that go into the judgement of the overall quality of a camera, but, other things being equal, I'd like to see improvements in these measurable aspects.